1. Field of the Invention
The present invention relates to steam generators in general and in particular to such steam generators of the Once-Through or "Benson" type also known as the Universal-Pressure boilers which have spiral and vertical flow furnace circuitry.
2. Description of the Related Art
The Universal Pressure boiler or the Once-Through boiler, derives its name from the fact that it was developed to be applicable functionally at all commercial temperatures and pressures, subcritical and supercritical. Numerically the boiler parameters and operating conditions are as follows:
Range in capacity, steam output about 300,000 lb/hr to an undetermined maximum exceeding 10,000,000 lb/hr.
Operating pressure subcritical, usually at 2400 psi constant throttle pressure, and supercritical, usually at 3500 psi constant throttle pressure. Variable operating pressure from the boiler feedwater pumps to the turbine throttle valves, over the operating load range is closely identified with the "Benson" boiler and its spiral furnace.
Steam and reheat temperatures as required, usually 1000 F. Constant steam temperature can be maintained to minimum once-through load with any fuel. Constant reheat temperature can be maintained to an intermediate load with any fuel, including coal, oil, or natural gas.
Operational control is completely automatic, including pumping and firing-rate control and steam temperature control.
The principle of operation is that of the once-through or "Benson" cycle. The working fluid is pumped into the unit as liquid, passes sequentially through all the pressure-part heating surfaces, where it is converted to steam as it absorbs heat, and leaves as steam at the desired temperature. There is no recirculation of water within the unit and, for this reason, a drum is not required to separate water from steam above the minimum once-through load or "Benson Point".
The furnace is completely fluid cooled. The flue gas side may be designed for balanced draft or pressure operation. Ash removal may be either dry or wet. Reheaters for single or two-stage reheat may be incorporated in the design for the reheat cycle.
Gas tempering, if used on pulverized coal fired units, controls gas temperatures entering the superheater and minimizes slagging in these surfaces. Reheater steam temperature is controlled by gas-recirculation, excess air, flue gas split flow control, and attemperation, separately or in combination.
The once-through boiler is designed to require a minimum flow inside the furnace circuits to prevent overheating of furnace tubes during all operating conditions. This flow must be established before start-up of the boiler. A bypass system, integral with the boiler, turbine, condensate and feedwater system, is provided so that the minimum design flow can be maintained through pressure parts which are exposed to high temperature combustion gases during the start-up operations and at other times when the required minimum flow exceeds the turbine steam demand. The minimum flow for startup may be established by a furnace recirculating pump or the boiler feedwater pump.
The bypass system includes steam-water separation equipment, such as a flash tank or vertical separator(s) and performs the following additional functions:
1. Controls the pressure and temperature of the steam leaving the boiler during start-up to conditions suitable for the turbine, condenser and auxiliary equipment. PA1 2. Provides means for recovering heat flowing to the bypass system, during start-up and low-load operations, utilizing the feedwater heaters or a furnace recirculating pump. PA1 3. Provides means for conditioning the water during start-up without delaying boiler and turbine warming operations. PA1 4. Protects the high temperature (secondary) super-heater against shock from water during start-up. PA1 5. Provides means for relieving excessive pressure in the system after a load trip.
Prior art variable operating pressure once through boilers or steam generators use furnace tube circuitry wherein the heated fluid flows from either vertical or spiral furnace tubing located around the total perimeter of the lower part of the generator to vertical tubing located in the upper part of the generator. With lower furnace spiral tubing construction, headers, bifurcates, trifurcates, bottles, or other fittings were located in the transition zone of the boiler and were used to split the fluid from the spiral tubes into a greater number of vertical tubes. Thus fluid flow leaving a spiral tube was usually split into two or more paths. Spiral tube connections at their upper termination in the furnace rear wall used a more extensive combination of headers and other fittings to split each spiral tube flow into multiple paths to form the construction for the furnace arch, front screen, pendent convection pass sidewalls, floor, and sometimes, the rear screen tube components. Furnaces recently equipped with all vertical tubing that operate variable pressure are commercially available, but their reliability and availability are currently being experienced. Furnaces with all vertical tubing can be designed with non-split flow circuitry for the upper front and sidewalls, but the rear wall flow is split to supply the various paths in the downstream upper furnace, e.g., the arch, screens, and pendant convection pass walls and floor.
It is necessary, when splitting a subcritical pressure steam-water mixture flow path as in the upper furnace tubing, that the downstream component receiving the split flow be designed for steam-water separation. This requires added fluid flow pressure loss to be built in to ensure for static stability. Tubes that receive all water, or a mixture with significantly less than average inlet steam quality, may not circulate sufficiently. The flow may stagnate, reverse, oscillate, or natural recirculation may occur within the component. All of these cause furnace materials to overheat if the pressure loss is not designed into the component to ensure sufficient forward circulation. The additional pressure loss required for static stability for the prior art designs caused higher plant heat rates due to the additional energy consumed by the boiler feedwater pump-drive. The additional pressure loss caused all upstream components, including the lower furnace, economizer, and feedwater system to the boiler feedwater pump to be designed for a higher pressure, and thus thicker walled pressure parts and additional costs. These split flow circuitry designs caused steam-water separation and resulted in some tubes receiving all steam, or a high quality steam mixture. The result caused the upper furnace, including the furnace outlet headers and the mechanical supports to be designed for an elevated temperature since the tubes receiving all steam absorbed enough heat to be significantly superheated over saturated steam thermodynamic conditions. The tube material has less strength at higher temperatures and hence they had to be thicker, more structurally supported, and upgraded to a higher strength alloy material. The upper furnace enclosure tubes and membranes, and screen tubes also had to be upgraded to a higher oxidation resistant alloy material due to the higher design temperatures.
The prior art designs were also non-drainable in some areas of the upper furnace rear wall circuitry, especially designs that attempted to minimize the number of intermediate headers or other fittings connecting the lower furnace tubes to the upper furnace tubes. Some portions of the upper furnace tube circuitry were constructed with pockets that did not drain water completely for maintenance purposes and became susceptible to out of service corrosion.
Thus it is seen that a fully drainable flow circuit was required which would reduce fluid flow pressure losses, and eliminate the heat transfer and material temperature levels and differentials caused by the split flow paths in the upper furnace rear wall circuitry of once-through boilers.